U.S. patent application number 12/403217 was filed with the patent office on 2009-09-17 for method for detecting current transfer in a plasma arc.
This patent application is currently assigned to Illinois Tool Works Inc.. Invention is credited to Alan A. Manthe, Anthony V. Salsich.
Application Number | 20090230098 12/403217 |
Document ID | / |
Family ID | 41061881 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090230098 |
Kind Code |
A1 |
Salsich; Anthony V. ; et
al. |
September 17, 2009 |
METHOD FOR DETECTING CURRENT TRANSFER IN A PLASMA ARC
Abstract
Methods and systems for transferring a plasma arc from between
an electrode and a tip to between an electrode and a workpiece and
back as dictated by the conditions at the cutting arc are provided.
The present disclosure allows for arc transfer detection without
use of a current sensor at the workpiece or knowledge of a precise
pilot circuit limit value through a novel plasma arc control
circuit. In one embodiment, the plasma arc control circuit provides
a programmable current source and a current sink configured to
limit current in a pilot arc control circuit. The pilot arc circuit
may be configured to signal its limiting status to a controller,
which may switch the pilot arc control circuit in or out of the
current path. Certain embodiments may include a pulse width
modulation control in the pilot arc control circuit for controlling
current flow through the pilot arc circuit.
Inventors: |
Salsich; Anthony V.;
(Appleton, WI) ; Manthe; Alan A.; (Hortonville,
WI) |
Correspondence
Address: |
FLETCHER YODER (ILLINOIS TOOL WORKS INC.)
P.O. BOX 692289
HOUSTON
TX
77269-2289
US
|
Assignee: |
Illinois Tool Works Inc.
Troy
OH
|
Family ID: |
41061881 |
Appl. No.: |
12/403217 |
Filed: |
March 12, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61036530 |
Mar 14, 2008 |
|
|
|
Current U.S.
Class: |
219/121.54 ;
219/121.39 |
Current CPC
Class: |
B23K 10/00 20130101;
B23K 9/067 20130101 |
Class at
Publication: |
219/121.54 ;
219/121.39 |
International
Class: |
B23K 9/00 20060101
B23K009/00 |
Claims
1. A plasma cutting system, comprising: a pilot arc circuit that
communicatively couples with a power supply, an electrode, and a
tip, wherein the pilot arc circuit, comprises: a pilot control
capable of controlling a current amplitude level through the pilot
arc circuit from the power supply to the tip by toggling a switch
to enable or disable current flow from the power supply to the tip
and creating a voltage potential between the tip and the work lead;
a power control capable of detecting when the switch is sustained
in a position to enable the current flow from the power supply to
the tip and capable of increasing the current output based on the
sustained position of the switch; and a cutting arc circuit that
communicatively couples with the power supply, the electrode, a
work lead, and the pilot arc circuit, wherein the cutting arc
circuit is capable of creating a voltage potential between the
electrode and the work lead.
2. The plasma cutting system of claim 1, wherein the pilot arc
circuit comprises a buck converter configured to limit the current
amplitude level at the tip.
3. The plasma cutting system of claim 1, wherein the pilot arc
circuit comprises a sensor and an inductor aligned in series
between the switch and the tip, wherein the sensor is configured to
provide a current feedback to a control circuit configured to
toggle the switch.
4. The plasma cutting system of claim 3, wherein the control
circuit comprises a hysteretic control circuit.
5. The plasma cutting system of claim 1, wherein the power supply
comprises an inverter.
6. The plasma cutting system of claim 1, wherein the cutting arc
circuit comprises a sensor and an inductor aligned in series
between the electrode and the power supply, wherein the sensor is
configured to provide a current feedback to the power supply.
7. A method of transferring a plasma arc of a plasma cutter,
comprising: providing current from a power supply to a pilot
control circuit, wherein the pilot control circuit is positioned
between the power supply and a tip; limiting a current amplitude
level to a maximum amplitude through the pilot control circuit by
toggling a switch to enable and disable current flow from the power
supply; and increasing a potential between a work lead and the tip
by limiting current flow through the pilot control circuit such
that excess current that does not flow through the pilot control
circuit is directed to the work lead.
8. The method of claim 7, comprising establishing a substantially
consistent arc that completes a circuit between the work lead and
the electrode.
9. The method of claim 7, comprising stabilizing current flow
between the electrode and the power supply with an inductor.
10. The method of claim 7, comprising limiting the current
amplitude level using a hysteretic control circuit configured to
toggle the switch based on a current feedback received from a
sensor positioned between the switch and the tip.
11. The method of claim 7, comprising stopping current flow through
the pilot control circuit when a level of current flow between the
work lead and the electrode is obtained.
12. The method of claim 11, comprising reinitiating current flow
through the pilot control circuit when an indication of arc failure
is identified.
13. The method of claim 7, comprising facilitating arc formation
between the electrode and a work piece coupled to the work
lead.
14. A plasma cutting system, comprising: a power supply capable of
providing multiple levels of current, wherein the power supply is
communicatively coupled to a tip and a work lead; a pilot arc
control circuit capable of limiting a level of current supplied to
the tip from the power supply to a level below a set point, wherein
the pilot arc control circuit, in operation, includes current flow
through a pilot arc between the tip and an electrode that is
communicatively coupled to the power supply to complete the pilot
arc control circuit; and a cutting arc control circuit capable of
detecting that a level of current being output by the power supply
is above the set point and that the pilot arc control circuit is
not performing a limiting function, which is indicative of current
flowing through the work lead and through a cutting arc to the
electrode.
15. The plasma cutting system of claim 14, wherein the pilot arc
control circuit comprises a hysteretic control circuit.
16. The plasma cutting system of claim 14, wherein the cutting arc
control circuit is capable of disabling flow through the pilot arc
control circuit when a sufficient level of current is determined to
be flowing through the work lead to the electrode.
17. The plasma cutting system of claim 14, wherein the work lead is
capable of coupling with a work piece.
18. The plasma cutting system of claim 14, wherein the power supply
comprises an inverter.
19. The plasma cutting system of claim 14, wherein the pilot arc
circuit comprises a buck converter.
20. The plasma cutting system of claim 14, wherein the pilot arc
control circuit and the cutting arc control circuit share common
components.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Non-Provisional Application of U.S.
Provisional Patent Application No. 61/036,530, entitled "Method for
Detecting Current Transfer in a Plasma Arc", filed Mar. 14, 2008,
which is herein incorporated by reference.
BACKGROUND
[0002] The invention relates generally to metal cutting systems,
and more particularly, to systems and methods for forming a first
plasma arc between an electrode and a tip of a plasma cutter then
transferring that arc such that it forms a second plasma arc
between the electrode and the work lead.
[0003] A plasma cutting system harnesses the energy in plasma
(e.g., high temperature ionized gas) to cut metal or other
electrically conductive material. Prior to cutting, the first
plasma arc, the pilot arc, is struck between the negatively charged
electrode and the tip of the plasma cutter. The arc must then be
transferred to the work piece to initiate cutting. The tip to work
potential determines the favorability of the plasma shift from the
tip to the workpiece and thus the transfer height (i.e., the height
at which the pilot arc will transfer and become the cutting arc) of
the system. Since a large transfer height is desirable, multiple
methods, such as the placement of resistors in series with the
pilot switch, are currently employed to increase the tip to work
potential. However, these methods fail to maximize transfer height
and often lead to lossy circuits.
[0004] After a pilot arc has been established, it is necessary to
detect that current will readily flow to the work piece so that
cutting current can be applied and the pilot circuit can be
disabled. Since the arc transfer is a critical step in the
initiation of plasma cutting, this requires a precise and accurate
measurement technique. Traditionally, a work current sensor, such
as a Hall-based current sensor, is connected to the work lead to
measure the current in the work lead prior to transfer. However, it
is now recognized that these sensors are costly and comprise a
large portion of the overall machine cost. Accordingly, it is now
recognized that there exists a need for plasma cutting systems
equipped to maximize transfer heights and tip to work potential
while minimizing cost.
BRIEF DESCRIPTION
[0005] The present disclosure is directed to systems and methods
relating to a plasma arc control circuit. One embodiment of the
present disclosure relates to arc transfer detection without use of
a current sensor in the work lead or knowledge of a precise pilot
circuit limit value. In particular, the present disclosure provides
methods and systems for transferring a plasma arc from between an
electrode and a tip to between an electrode and a workpiece and
back as dictated by the conditions at the cutting arc. In one
embodiment, the plasma arc control circuit provides a programmable
current source and a current sink configured to limit current in a
pilot arc control circuit. The pilot arc circuit may be configured
to signal its limiting status to a controller, which may switch the
pilot arc control circuit in or out of the current path. Certain
embodiments may include a pulse width modulation control in the
pilot arc control circuit for controlling current flow through the
pilot arc circuit.
DRAWINGS
[0006] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0007] FIG. 1 is a perspective view of an exemplary plasma cutting
power supply unit in accordance with aspects of the present
disclosure;
[0008] FIG. 2 is a circuit diagram illustrating an exemplary
embodiment of the power supply circuitry in accordance with aspects
of the present disclosure;
[0009] FIG. 3 is a circuit diagram illustrating one embodiment of
the power supply circuitry in accordance with aspects of the
present disclosure;
[0010] FIG. 4 is a block diagram illustrating exemplary processing
logic that may be used to control the current source output and the
pilot control circuitry in accordance with aspects of the present
disclosure;
[0011] FIG. 5 is a block diagram illustrating exemplary logic that
may be used to establish the pilot arc and the cutting arc in
accordance with aspects of the present disclosure;
[0012] FIG. 6 is a graphical representation of exemplary current
waveforms through the tip, the electrode, and the work piece during
cutting arc initiation in accordance with aspects of the present
disclosure;
[0013] FIG. 7 is a graphical representation of exemplary voltage
potential waveforms during cutting arc initiation in accordance
with aspects of the present disclosure;
[0014] FIG. 8 is a block diagram illustrating exemplary logic that
may be used to transfer the cutting arc back to the pilot arc in
accordance with aspects of the present disclosure;
[0015] FIG. 9 is a graphical representation of exemplary current
waveforms through the tip, the electrode, and the work piece during
transfer back to the pilot arc from the cutting arc in accordance
with aspects of the present disclosure; and
[0016] FIG. 10 is a graphical representation of exemplary voltage
potential waveforms during transfer back to the pilot arc from the
cutting arc in accordance with aspects of the present
disclosure;
DETAILED DESCRIPTION
[0017] FIG. 1 illustrates an exemplary plasma cutting power supply
10, which powers, controls, and provides consumables to a cutting
operation in accordance with aspects of the present disclosure. A
torch 14 and a work lead clamp 16 are communicatively coupled to
the power supply unit 10 and may be utilized to perform cutting
operations. The front side of the power supply unit 10 in the
illustrated embodiment contains a control panel 12, through which a
user may control the supply of materials, such as power, gas flow,
and so forth, to the cutting torch 14. The work lead clamp 16
typically connects to a workpiece to close the circuit between the
torch 14, the work piece, and the supply unit 10, and to ensure
proper current flow. The portability of the unit 10 depends on a
handle 18, which enables the user to move the power supply unit 10
to the location of the workpiece.
[0018] Internal components of the power supply unit 10 receive
power from a wall outlet, a generator, a battery, or the like and
then supply power to circuitry that enables the formation of plasma
arcs necessary for the plasma cutting operation. FIG. 2 illustrates
an arc control circuit 20 that controls the formation of the plasma
arcs between a workpiece 22, a torch tip 24, and an electrode 26 by
controlling current flow through circuit components. A first
current path through a pilot arc circuit may be established when a
programmable current source 28 outputs a first current 30 that
flows through a node 32 into a current regulator 34, which is
capable of programmable switching. The current regulator 34 outputs
a second current 36 (e.g., pulse width modulated current). In one
embodiment, the pulse width modulation of the second current 36 is
hysteretic, wherein hysteretic means maintaining current between a
lower limit and an upper limit. The second current 36 flows through
the tip 24 and the electrode 26, establishing a pilot arc 38
between the tip 24 and the electrode 26. The second current 36 then
returns to the current source 28, completing the first current path
through the components that define the pilot arc circuit. A second
current path through a cutting arc circuit may be established when
the programmable current source 28 outputs the first current 30
that flows through a node 32 into the workpiece 22 and the
electrode 26, establishing a cutting arc 40 between the workpiece
22 and the electrode 26. The first current 30 then returns to the
current source 28, completing the second current path through the
components that define the cutting arc circuit. The amount and path
of the current flow through the components of the arc control
circuit 20 define a voltage potential 42 between the electrode 26
and the tip 24, a voltage potential 44 between the tip 24 and the
workpiece 22, and a voltage potential 46 between the electrode and
the workpiece 46.
[0019] In one embodiment, the arc control circuit 20 achieves a
current limit through the pilot arc circuit by employing a chopper
switch in the current regulator 34. In one embodiment, the current
source 28 is configured to programmably provide a range of output
currents limited only by its rated output voltage. The current
regulator 34 may comprise a fixed current limiter and may be
switched in or out of the active circuit to achieve the current
limit through the pilot arc circuit. The chopper switch in the
current regulator 34 may be kept in an ON state as long as the
pilot current is less than a preset level, where the ON state may
be defined by a closed switch position that allows current flow
through the pilot arc circuit, establishing the pilot arc 38. If
the current exceeds the preset level, the chopper switch toggles to
an OFF state, where the OFF state is defined by an open switch
position that prohibits additional current flow through the pilot
arc circuit. When the switch is in an OFF state, the current decays
to a lower limit at which point the switch closes to an ON state to
maintain the pilot arc 38. In this way, the current regulator 34
switches ON and OFF to control the current through the pilot arc
circuit and maintain the pilot arc 38.
[0020] After a current flow and a pilot arc 38 have been
established in the pilot arc circuit, the setpoint of the current
source 28 can be incrementally increased until the chopper switch
in the current regulator 34 starts to switch ON and OFF to maintain
a preset level of current in the pilot arc circuit by limiting the
amount of current allowed to flow from the current source 28
through the pilot arc circuit. In one embodiment, when the current
source 28 receives feedback indicating that limiting is occurring,
no substantial transfer of the pilot arc 38 to the workpiece 22 is
occurring. However, when the chopper switch in the current
regulator 34 stays ON as the current 30 from the current source 28
is increased, this indicates that transfer of the pilot arc 38 to
the workpiece 22 is occurring since the current 30 from the source
28 is configured to flow either to the workpiece 22 or to the
current regulator 34 when exiting the node 32. The current
regulator 34 may be left in the current path while the current
output 30 is increased to a preset level without the pilot arc
circuit going into limit. At this point, the cutting arc 40 is
established between the electrode 26 and the workpiece 22, the
current regulator 34 may be removed from the current path, and
cutting may occur. During the plasma cutting operation, if imminent
cutting arc 40 outage is detected, the current regulator 34 may be
placed back in the current path, re-enabling current flow through
the pilot arc circuit and reestablishing the pilot arc 38.
[0021] The combined use of the pilot arc circuit with the cutting
arc circuit in accordance with aspects of the present disclosure
offers distinct benefits. For instance, there is no need for a
current sensor at the workpiece 22 for detection of arc transfer to
the workpiece 22. The exact preset current limit in the current
regulator 34 need not be known. Instead, when no arc transfer from
the pilot arc 38 to the cutting arc 40 occurs, the programmable
current output 30 from the current source 28 may be manipulated to
find a threshold for the limit value of the current regulator 34.
Additionally, any time the current regulator 34 is in limit, an
improved voltage potential 44 is established between the tip 24 and
the workpiece 22, leading to an advantageous transfer height.
[0022] FIG. 3 illustrates one embodiment of the arc control circuit
shown in FIG. 2. In this embodiment, a first current path through
the pilot arc circuit may be established when the programmable
current source 28 outputs the first current 30 that flows through
the node 32, a first transistor switch 48, a first current sensor
50, a first inductor 52, the tip 24, and the electrode 26, forming
the pilot arc 38 between the tip 24 and the electrode 26 with the
resulting second current 36 (i.e., pulse width modulated current).
The current 36 then passes through a second current sensor 54,
which provides a feedback signal 56 to the current source 28. The
current continues through a second inductor 58 to return to the
current source 28, thus completing the first current path through
the pilot arc circuit. The pilot arc circuit also contains a diode
60 and a second transistor switch 62, which breaks the path through
the diode 60. The diode 60 and the first transistor switch 48
combine with the first inductor 52 to form and function as a buck
converter. The intrinsic property of the first inductor 52 that
attempts to keep current flow constant is exploited. A current
feedback signal 64 from the first current sensor 50 communicates
with a pulse width modulation control 66, which switches the first
transistor 48 ON and OFF to maintain the pilot arc 38. When the
first transistor 48 is ON (i.e. in a closed position), the first
inductor 52 resists increases in current flow and energy builds in
the first inductor 52. When the first transistor switch is OFF
(i.e. in an open position), the current through the pilot arc
circuit is forced by the first inductor 52 to freewheel through the
tip 24, the electrode 26, and the second inductor 58, up through
the diode 60 and the second transistor 62, and back through the
first inductor 52. In this way, the buck converter (i.e. the first
transistor 48, the diode 60, first inductor 52, and the second
inductor 58) controls the current flow in the tip 24, preventing
current overshoots and subsequent tip 24 damage. As previously
described with respect to FIG. 2, a second current path through a
cutting arc circuit may be established when the programmable
current source 28 outputs a first current 30 that flows through a
node 32 into the workpiece 22 and the electrode 26, establishing a
cutting arc 40 between the workpiece 22 and the electrode 26. The
first current 30 then returns to the current source 28 through a
second current sensor 54 and a second inductor 58, completing the
second current path through the components that define the cutting
arc circuit. A first capacitor 68, a second capacitor 70, and a
ground 72 minimize circuit noise. Additionally, the first capacitor
68 and the second capacitor 70 may provide a high speed path for
current flow when the first transistor 48 is switching.
[0023] FIG. 4 is a block diagram illustrating exemplary processing
logic that may be used to control the functioning of the arc
control circuit 20 by controlling the current source output 30 and
the pilot control circuitry. In the illustrated embodiment, a
controller 74 comprises a pilot controller 76, a main controller
78, and a processor 80, which receive feedback signals from and
deliver commands to the plasma cutting operation. The pilot
controller 76 and the main controller 78 may comprise software,
memory, circuitry, and so forth. The pilot controller 76 may
receive signals regarding the functioning of the pilot arc circuit,
such as a limiting status 82 of the current regulator 34, and
output a control signal 84 based on its inputs. Similarly, the main
controller 78 may receive signals regarding the current source 28,
such as a level of the current output 86, and output a control
signal 88 based on its inputs. The processor 80 receives the
control signals 84, 88 from the pilot controller 76 and the main
controller 78 and integrates the information with any additional
auxiliary input signals 90. The processor then generates output
control signals that drive the operation of the arc control circuit
20. The pilot arc circuit is controlled by a signal 94 from the
processor 80 that enables or disables the pilot control circuit. A
signal 96 from the processor 80 drives the increase or decrease of
output current from the current source 28. Additionally, the
processor may output one or more auxiliary signals 98 that drive
peripheral functions related to the plasma cutting operation.
[0024] FIG. 5 is a block diagram illustrating exemplary logic
behind one embodiment of the present disclosure that may be used to
establish the pilot arc 38 and the cutting arc 40. Each block in
FIG. 5 may represent a function or step. First, in the illustrated
embodiment, the controller 74 initiates the arc start, as
represented by block 100, and enables the pilot control circuitry,
as represented by block 102. Initially, the main controller 78
outputs a signal 88 that commands a low output current level, as
represented by block 104. The processor then outputs an auxiliary
control signal 98 to enable torch pressure, as represented by block
106. Feedback regarding whether or not the pilot control is
limiting is then sought from the arc control circuit 20, as
represented by block 108. If the pilot control is limiting, the
controller 74 maintains a constant output current level. If the
pilot control is not limiting, the main controller 78 outputs a
control signal 88 that incrementally increases the current output
of the current source 28, as represented by block 110. Feedback
regarding whether the main controller 78 has reached a defined
current setpoint is then sought, as represented by block 112. If
the main controller 78 has not reached the defined setpoint,
feedback is once again sought regarding whether or not the pilot
control is limiting, as represented by block 108. If the main
controller 78 has reached the defined setpoint, current is flowing
through the work piece 22, the pilot controller 76 is disabled, and
the main control ramps up the current output to a level sufficient
for the plasma cutting operation, as represented by block 114. At
this point, as represented by block 116, the cutting arc 40 is
cutting. The described pilot-to-work transfer, as represented by
block 118, illustrates the logic behind the circuit illustrated in
FIG. 2.
[0025] FIGS. 6 and 7 illustrate exemplary current and voltage
potential waveforms, respectively, from when the pilot arc 38 is
initially struck until arc transfer from between the electrode 26
and the tip 24 to between the electrode 26 and the workpiece 22.
FIG. 6 illustrates a tip current waveform 120, an electrode current
waveform 122, and a work current waveform 124. FIG. 7 illustrates a
work-electrode potential waveform 126, a tip-electrode potential
waveform 128, and a work-tip potential waveform 130. The arc
control circuit 20 begins operation at an initial start time 132,
beginning current flow through the tip 24 and the electrode 26. At
a later time 134, air flow through the welding system begins,
giving rise to a work-electrode potential and a tip-electrode
potential. Subsequently, at a time 136, the pilot circuit goes into
limit, leveling out current flow through the tip 24 and giving rise
to an increase in the work-electrode potential and an initiation of
a tip-work potential. At a later time 138, tip current decreases
while work current increases due to a diversion of current from the
tip 24 to the workpiece 22. At this time 138, the work-electrode
potential decreases to a new steady state value, and the tip-work
potential falls back to zero. Subsequently, at a time 140, the
pilot circuit is disengaged by the controller 74, triggering a
falloff of tip current down to zero and a corresponding increase in
work current.
[0026] FIG. 8 is a block diagram illustrating exemplary logic
behind one embodiment of the present disclosure that may be used to
transfer the cutting arc 40 back from between the workpiece 22 and
the electrode 26 to between the tip 24 and the electrode 26 during
instances when an imminent arc outage may be detected. In this
diagram, the logic 118 behind the pilot-to-work arc transfer
remains the same with respect to FIG. 5. However, once the cutting
arc is cutting, as represented by block 116, a feedback signal
regarding whether or not arc outage is imminent is sought, as
represented by block 142. If arc outage is not imminent, cutting
continues, as represented by block 116. If arc outage is imminent,
the pilot controller is enabled, as represented by block 102, to
reestablish the pilot control circuitry as part of the current
path. The main controller 78 outputs a control signal 88 to
incrementally reduce the output current, as represented by block
144. A feedback signal is then sought from the arc control circuit
20 regarding whether or not the pilot control is limiting, as
represented by block 108. If the pilot control is limiting, the
main controller 78 outputs a control signal 88 to incrementally
reduce the output current, as represented by block 144. If the
pilot control is not limiting, the pilot to work transfer logic
represented by block 118 is employed to once again transfer the
pilot arc 38 from between the electrode 26 and the tip 24 to
between the electrode 26 and the workpiece 22.
[0027] FIGS. 9 and 10 illustrate exemplary current and voltage
potential waveforms, respectively, from when the cutting arc 40 is
cutting to when cutting arc outage is imminent to when the pilot
arc 38 is reestablished. FIG. 9 illustrates a tip current waveform
146, an electrode current waveform 148, and a work current waveform
150. FIG. 10 illustrates a work-electrode potential waveform 152, a
tip-electrode potential waveform 154, and a work-tip potential
waveform 156. Initially, the cutting current is established and
flowing through the electrode and the workpiece and a
work-electrode and a tip-electrode potential exist as indicated by
arrow 158. However, when an arc outage becomes imminent, as
indicated by an increase in the work-electrode and the
tip-electrode potentials designated by arrow 160, the pilot circuit
is reengaged, as indicated by arrow 162, to prevent loss of the
plasma arc. When the pilot circuit is reengaged, as indicated by
arrow 162, the tip current increases until the pilot circuit goes
into limit and the tip current becomes limited, as indicated by
arrow 164. Additionally, when the pilot circuit goes into limit,
the work-electrode potential and the tip-electrode potential
decrease from a peak while the main output ramps down, as indicated
by arrow 166 and the electrode current spikes downward. For a short
time duration, the output current briefly undershoots the tip
limit, as indicated by arrow 168. When the pilot circuit is back in
limit, the work-electrode and the tip-electrode potential fall back
to a steady state value.
[0028] While only certain features of the present disclosure have
been illustrated and described herein, many modifications and
changes will occur to those skilled in the art. It is, therefore,
to be understood that the appended claims are intended to cover all
such modifications and changes as fall within the true spirit of
the present disclosure.
* * * * *